Although the effects of extrinsic barriers to dispersal have increasingly been shown to
play a large role in the structuring of contemporary genetic diversity, describing the
relationship between landscape structure, stochastic disturbance, and genetic diversity
remains a major challenge. Here, environmental features for 27 barrier-isolated
populations (2,232 individuals) of coastal cutthroat trout from western Oregon are
compared with data from seven microsatellite loci to examine how watershed-scale
environmental factors shape genetic diversity. Isolated headwater populations of
coastal cutthroat trout are strongly differentiated (mean Fst =0.33) but intrapopulation
microsatellite genetic diversity (mean number of alleles per locus = 5, mean He =
0.60) was only moderate. Differences in genetic diversity of fish from the Coast
Range (mean alleles = 47) and Cascade Mountains (mean alleles = 30) (P = 0.02) coincided with differences in regional landscape feature. Furthermore, scatter evident
from isolation by distance plots within ecoregions indicated that population structure
was primarily mediated by gene flow in the Coast Range, but in the Cascade
Mountains, genetic drift the dominant factor influencing genetic patterns. Thus
through comparisons between landscape structure and genetic diversity we
demonstrate an example where physical landscape features play a substantial role in
the structuring of genetic diversity
